Question

What do you mean by successive electron gain enthalpies? Why is the second electron enthalpy of an atom positive?

Answer 1

Electron gain enthalpy is the amount of energy released when an isolated gaseous atom or ion accepts an electron to form a negative gaseous ion which is called an anion. This anion when it accepts another electron to form an anion with two negative charges and releases some amount of energy then, it is known as successive electron gain enthalpy.

Complete answer:
We can define electron gain enthalpy as the amount of energy released when an isolated gaseous atom or ion accepts an electron to form a negative gaseous ion which is called an anion.
Successive electron gain enthalpies can be defined as when an anion accepts the electron to form an anion with two negative charges and releases some amount of energy. If this anion accepts another electron to form an anion with three negative charges, it is known as third electron gain enthalpy and so on.
Now we can say that the second electron gain enthalpy for any atom is always positive because there will be more number of electrons in the previously formed anion than the number of protons and hence, the incoming electron will face electronic repulsions. Therefore, energy will be given to the system to accept electrons.

Note:
We should not get confused between the electron gain enthalpy and electron affinity. Electron gain enthalpy is the amount of energy released when an isolated gaseous atom or ion accepts an electron to form a negative gaseous ion which is called an anion while electron affinity is the tendency of an atom to pull the electron density towards itself.